3. The writer suggested in 1897[221] that the light acts chemically in the heliotropic reactions and in 1912 that the heliotropic reactions probably follow the law of Bunsen and Roscoe,[222] and it was possible to confirm this idea by direct experiments.[223] This law states that the photochemical effect of light equals _i t_ where _i_ is the intensity of the light and _t_ the duration of illumination. The experiments were carried out on young regenerating polyps of _Eudendrium_ by measuring the time required to cause fifty per cent. of the polyps to bend to the source of light. The intensity of light was varied by altering the distance of the source of light from the polyps. Table VI gives the result. [221] Loeb, J., _Arch. f. d. ges. Physiol._, 1897, lxvi., 439. [222] Loeb, J., _The Mechanistic Conception of Life_, Chicago, 1912, p. 27. [223] Loeb, J., and Ewald, W. F., _Zentralbl. f. Physiol._, 1914, xxvii., 1165. TABLE VI ------------+---------------------------------------------- _Distance |_Time Required to Cause Fifty Per Cent. of the between | Polyps to Bend towards the Source of Light_ Polyps and +----------------------+----------------------- Source of | Observed | Calculated from Light_ | | Bunsen-Roscoe Law ------------+----------------------+----------------------- _Metres_ | _Minutes_ | _Minutes_ ------------+----------------------+----------------------- 0.25 | 10 | 0.50 | between 35 and 40 | 40 1.00 | 150 | 160 1.50 | between 360 and 420 | 360 ------------+----------------------+----------------------- We must therefore conclude that the heliotropic curvature of the polyps is determined by a photochemical action of the light. The light brings about or accelerates a chemical reaction which follows the Bunsen-Roscoe law. As soon as the product of this reaction on one side of the polyp exceeds that on the other by a certain quantity, the bending occurs. When the product _it_ is the same for symmetrical spots of the organism no bending can result. This is what our theory suggested. It is very difficult to prove directly the applicability of the Bunsen-Roscoe law for free-moving animals, but it can be shown that intermittent light is as effective as constant light of the same intensity, provided that the total duration of the illumination by the intermittent light is equal to that of the constant light, and the duration of the intermission is sufficiently small (Talbot’s law). Talbot’s law is in reality only a modification of the Bunsen-Roscoe law. Ewald has proved in a very elegant way the applicability of Talbot’s law to the orientation of the eyestalk of _Daphnia_.[224] This makes it probable that the law of Bunsen-Roscoe underlies generally the heliotropic reaction of animals.